SILLY, weakly foolish, stupid. This is the current sense of a word which has much changed its meaning. The O.E. scdig (usually ges&lig) meant prosperous, happy, and was formed from sal, time, season, hence happiness, cf. Icel. scda, bliss; Ger. selig, blessed, happy, etc., probably also allied to Lat. sahus, whole, safe. The development of meaning is happy, blessed, innocent or simple, thence helpless, weak, and so foolish. The old provincial and Scottish word for a caul (q.v.) was " sillyhow," i.e. " lucky cap." The development of meaning of " simple," literally " onefold " (Lat. simplex), plain, artless, hence unlearned, foolish, is somewhat parallel. A special meaning of " simple," in the sense of medicinal herbs, is due to the supposition that each herb had its own particular or simple medicinal value.
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)